Conventional fluid flow measurement devices such as described in U.S. Pat Nos. 4,397,194; 4,217,777; 4,202,211; and 3,498,290 use ultrasonic techniques for measuring fluid flow. The 4,397,194 patent discloses an ultrasonic flow meter wherein an acoustic pulse is transmitted by one transducer and propagated to the fluid at an angle to the longitudinal flow axis and received by another transducer. The time delay difference between the upstream and downstream transit times which depends on full velocity was determined to provide a flow rate reading.
The 4,217,777 patent discloses a sonic detector having a head which is positionable at different distances from the base of a channel and provides data signals corresponding to the transit time for sonic signals between the top of the liquid in the head, which signals are variable with the level of the liquid in the channel. A liquid velocity detector provides data signals which vary with the velocity of the liquid flowing through an area in the channel.
The 4,202,211 patent discloses a non-invasive system for measuring liquid flow through channels. The system utilizes an ultrasonic Doppler velocity detector which is positionable in a space below the channel and provides data signals which vary with the velocity of the liquid flow through an area in the channel. The system also utilizes a unit which responds to the variable data signals and the constant data signals to provide an output representing the flow through the channel in terms of the product of the area of the cross-sectional profile of the channel through which the fluid is flowing and the velocity of the fluid.
The 3,498,290 patent discloses a volumetric blood flow meter in which a first transducer locates and measures the diameter of a vessel and a second transducer determines the velocity of blood in the vessel by Doppler frequency shift techniques. This flow meter measures volumetric blood flow in an undisturbed vessel of a patient from the surfaces of his body.
U.S. Pat. No. 4,254,482 discloses the use of sonic signals for measuring the flow rate and liquid level of a fluid flowing in a channel. An echo location system operates to detect returns of reflected sonic signals from the surface of the liquid in a channel during successive transmission cycles.
Another, more accurate, technique for measuring fluid flow is accomplished through the invasive use of microwave radiation.
U.S. Pat. No. 4,167,736 discloses a method of using microwave radiation and Doppler frequency shift techniques to measure fluid flow. This is accomplished by generating a microwave frequency electrical signal, passing the signal to an aerial for radiation into the fluid, detecting the radiated signal after reflection by the fluid, determining from the radiated and reflected signals the Doppler frequency shift in between as a measure of the fluid flow rate. This type of monitoring is done through obtrusive contact with the fluid.
Although the use of microwave radiation is useful as an accurate measurement tool, the invasive nature of the method and equipment makes its use for contaminant-laden fluids, such as circulating machine tool coolant, undesirable. Such equipment becomes rapidly coated with contaminant and the accuracy of the measurements is eliminated.